[Coral-List] Vol 43, Issue 25The myth of 100% coral cover (Eugene Shinn)

[Gregor Hodgson]

 100% is a bit of a red herring. Far more important questions are: How has
hard coral cover changed over time? Do more or fewer reefs have "high" coral
cover now than 20 years ago?

But back to the red herring -- as Gene has implied, the answer is not quite
as obvious as many would think, and will depend greatly on what method is
used and how "reef" is defined. Whether in 1960 or 5000 years ago, the issue
is one of scale and size ‹ akin to the question regarding the length of the
coast of Sweden? How big does a reef area need to be before it can be
considered "a coral reef 100% covered in coral ‹ 10 sq m or  100 m x 10 m or
10 km x 2 km?" How about the spaces between the corals or between the
branches of e.g. Acropora? What if there are other organisms such as algae,
sponges or zooanthids occupying some substrate. Does that disqualify the
100% designation? If so, then few reefs could qualify as 100% coral.

As we are all well aware, because of natural variations in substrate type,
water motion and the movement and deposition of sand and rubble, most coral
reefs have always been composed of areas of coral separated by areas of
silt, sand and rubble. All reefs have zones ‹ with differing levels of coral
cover from zero on up. So on the scale of kilometers or miles of reef (even
ignoring interstitial organisms), few reefs (taken in total)  anywhere could
be considered 100% covered in coral. As the scale, decreases to 100s or 10s
of meters, and if we ignore the odd interstitial benthic organism growing in
between hard corals, there have been and continue to be many "reefs" around
the world that could be considered to be 100% covered in coral.

Using a standardized method to measure % substrate is obviously very
important in tracking changes in coral cover. Thanks to thousands of Reef
Check volunteers, who drop a 3 mm sampling point every 0.5 m   along 160 m
of transect line ‹ we have a comparable dataset of coral cover from 1997.
This has been used in several key papers examining coral cover and other
issues. Reef Check data are available free to any researcher.

 Eakin CM, Morgan JA, Heron SF, Smith TB, Liu G, et al. (2010) Caribbean
Corals in Crisis: Record Thermal Stress, Bleaching, and Mortality in 2005.
PLoS ONE 5(11): e13969. doi:10.1371/journal.pone.0013969
<http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013969>

  Selig ER, Bruno JF (2010) A Global Analysis of the Effectiveness of Marine
Protected Areas in Preventing Coral Loss. PLoS ONE 5(2): e9278. doi:10.1371/
 journal.pone.0009278 <http://reefcheck.org/PDFs/Bruno2010.pdf>

   John F. Bruno, Hugh Sweatman, William F. Precht, Elizabeth R. Selig,
Virginia G.   W. Schutte (2009) Assessing evidence of phase shifts from
coral to macroalgal   dominance on coral reefs. Ecology: Vol. 90, No. 6, pp..
1478-1484.  <http://www.esajournals.org/doi/full/10.1890/08-1781.1>

It is important to realize, however, that one of the shifts in our shifting
baseline is the fact that a lot of "reefs"  or reef zones in the Caribbean
have completely disappeared and are no longer monitored except in locations
that continue to use permanently marked transect locations. When Acropora
pretty much died out in the Caribbean, two zones previously found on those
reefs either disappeared or changed into rock. While the Acropora
cervicornis reefs that used to occupy wide swathes of reefs at mid-depths
are entirely gone in most of the Caribbean, the dead skeletons of A. palmata
reefs can still be observed along miles of reefs in many countries. Most
Reef Checkers and other researchers tend now to monitor reefs with living
corals. This issue has not been taken into account in most recent attempts
to analyze trends in coral cover in the Caribbean. If the previously living
Acropora reef areas were included in monitoring and calculations, then my
guess is that the current coral cover percentages reported for many
Caribbean reefs would be 20 to 30% lower.

Gregor Hodgson, PhD
Executive Director
Reef Check Foundation
Pacific Palisades, CA 90272 USA
Gregorh at reefcheck.org